1. Field of the Invention
This invention relates generally to plasma processing systems, and more particularly to apparatus and methods for stabilizing interactions between plasmas and power delivery systems.
2. Brief Description of the Prior Art
Plasma processing systems are widely used in a variety of industries for modifying the surface properties of materials. For example, the manufacture of modern integrated circuits generally involves many processing steps that use plasmas for etching of submicrometer features, or for depositing atomically thin layers of materials.
A typical plasma processing system comprises a processing chamber and a power delivery system that creates and maintains the plasma inside the chamber. Electrically, the plasma is a load with a characteristic impedance that is driven by the power generator. The impedance of a processing plasma is generally not constant, however, but may vary depending upon process conditions or other variables. Variations in plasma impedance may adversely affect the power delivery from the generator, which typically provides optimal power delivery only for a particular load impedance. These variations may also result in undesired drifts or perturbations in process variables, such as etch or deposition rates, due to changes in the physical properties of the plasma at different power levels. As a result, plasma processing systems are often equipped with impedance matching and control mechanisms or circuitry that respond to changes in plasma impedance and maintain desired levels of power delivery to the plasma.
The use of impedance matching systems and control circuitry is not always sufficient to ensure stability of the plasma in a plasma processing system, particularly in operating regimes where plasma properties fluctuate rapidly or exhibit nonlinear behaviors. U.S. Pat. No. 5,441,596, for example, describes a method of stabilizing power furnished to a plasma by engaging an impedance matching network only after the plasma has been ignited and stabilized at an initially low power level. Others in the field have addressed problems of plasma instability by looking to improvements in the speed and sophistication of matching network technology, as described for example in U.S. Pat. No. 6,313,584. In processing applications powered by modern switch-mode power generators, plasma stability may also be enhanced by incorporating circuitry that absorbs and dissipates energy at frequencies distant from the fundamental frequency of the power furnished by the generator, as described for example in U.S. Pat. No. 5,747,935.
Despite these improvements, problems of plasma instability in many semiconductor fabrication operations remain, particularly those involving the use of electronegative process gases. This is due in part to the trend toward reduced feature size of semiconductor products, which often requires process operations at reduced power levels and gas pressures for critically dimensioned features. In these process regimes, fluctuations or oscillations in plasma properties may occur due to competing physical interactions among the plasma constituents, as for example fluctuations in ion density due to time-varying rates of ionization and electron attachment in a plasma formed of an electronegative gas. See, e.g., M. A. Lieberman et al., “Instabilities in low-pressure inductive discharges with attaching gases,” 75 Applied Physics Letters 23 (Dec. 6, 1999) 3617–19; A. Descoeudres et al., “Attachment-induced ionization instability in electronegative capacitive RF discharges,” Plasma Sources Sci. Technol. 12 (2003) 152–57. As a result, the impedance of the plasma can become strongly dependent upon power level and may even exhibit negative impedance characteristics; that is, an increase in current will result in a decrease in voltage across the plasma, causing even more current to be conducted through the plasma. When attempting to operate in these low power and pressure regimes, unstable voltage oscillations may occur within the plasma that are beyond the capabilities of the power supply or impedance matching control loops to counteract, resulting in uncontrollable and unrepeatable variations in process parameters.
Plasma instability phenomena encountered in low power and pressure process regimes may be further compounded by the electrical characteristics of modern switch-mode power supplies. It has been observed that in a given process regime, plasma instabilities may result when powering the plasma with certain radio frequency (RF) power generators but not others. As a general matter, processes that employ modern switch-mode power supplies to power the plasma are found to be more susceptible to plasma instabilities at low powers and pressures, due primarily to the increased sensitivity of the open-loop power output of these compact, high efficiency generators to the impedance of the plasma load compared to that of generators based on older linear technology. When the open-loop power output of a power generator varies significantly with load impedance, the generator may interact with the plasma load in a way such that fluctuations in plasma impedance are reinforced or amplified by the delivery system, resulting in unstable oscillations in plasma properties and a detrimental impact on the process. These fluctuations may in some cases be exacerbated by unsuccessful actions of the generator power control loop to correct for the power variances, or may occur outside the control bandwidth of the generator altogether.
A conventional approach to problems of plasma instabilities at low powers and pressures has been simply to avoid operation in those regimes, or to alter other process parameters, such as gas flow rates and ratios, in order to stabilize the plasma. U.S. Pat. No. 6,399,507, for example, describes a method of extending the stability of a processing plasma into low power regimes by increasing gas pressures and apportioning the power furnished to the plasma between capacitive and inductive components. In some cases, plasma stabilization has been demonstrated using high bandwidth, high gain feedback control of the plasma RF generator, as described for example in D. L. Goodman and N. M. P. Benjamin, “Active control of instabilities for plasma processing with electronegative gases,” J. Phys. D: Appl. Phys. 36 (2003) 2845–2852. Many process engineers have also observed that adding particular lengths of transmission line between the power generator and process chamber has the effect in some cases of stabilizing an otherwise unstable plasma process. Use of this technique has been largely empirical and ad hoc, however, and procedures for determining how and when it may be effective have not heretofore been offered. Indeed, process engineers today are taught as a general matter to avoid the use of transmission lines or of otherwise introducing electrical delays into the coupling between a power generator and the plasma it powers (see, e.g., U.S. Pat. No. 5,643,364). Moreover, a growing trend in plasma processing systems toward direct mounting of the power generator on the chamber is removing the transmission line as an available means of stabilizing the plasma in unstable regimes.
It would be desirable if the stability of processing plasmas could be extended into otherwise unstable processing regimes without the need to alter process parameters, or to search by trial and error for particular lengths of transmission lines that stabilize the plasma. It would be further desirable to provide a means of stabilizing processing plasmas not only in a single operating condition, but under a range of useful process regimes.